This invention relates to rapid pressure swing adsorption (RPSA) processes, and more particularly to hydrogen production via RPSA processes.
The increasing demand for hydrogen, particularly in petroleum refining and processing has provided a strong economic motivation to develop processes to recover hydrogen from refinery fuel gas, coke oven gas and other similar sources as well as from more traditional sources such as reformer off-gas. For most applications, a high purity hydrogen product is required.
The process of production and recovery of hydrogen by steam and/or air reforming of hydrocarbon rich gas streams, such as natural gas, naphtha, or other mixtures of low molecular weight hydrocarbons, is well known in the art. Typical commercial sources for the production of hydrogen include reforming of natural gas or partial oxidation of various hydrocarbons. The reforming is carried out by reacting the hydrocarbon with steam and/or with oxygen-containing gas (e.g., air or oxygen-enriched air), producing a hydrogen gas stream containing accompanying amounts of oxides of carbon, water, residual methane and nitrogen. Unless recovery of carbon monoxide is desired, the carbon monoxide is customarily converted to carbon dioxide by water gas shift reaction to maximize the hydrogen content in the stream. Typically, this gas stream is then sent to a PSA unit. Other hydrogen-rich gas sources that can be upgraded by PSA technology to a high purity product include refinery off-gases with C1-C6 hydrocarbon contaminants. See, e.g., U.S. Pat. No. 3,176,444 to Kiyonaga.
In PSA processes, a multi-component gas is passed to at least one of a plurality of adsorption beds at an elevated pressure to adsorb at least one strongly adsorbed component while at least one relatively weakly adsorbed component passes through. In the case of hydrogen production via pressure swing adsorption (H2 PSA), H2 is the weakly adsorbed component, which passes through the bed. See, e.g., U.S. Pat. No. 3,430,418 to Wagner, U.S. Pat. No. 3,564,816 to Batta and U.S. Pat. No. 3,986,849 to Fuderer et al. At a defined time, the feed step is discontinued and the adsorption bed is depressurized in one or more steps, which permit essentially pure H2 product to exit the bed. Then a countercurrent desorption step is carried out, followed by countercurrent purge and repressurization.
H2PSA processes for the production of high purity H2 (99.9% and higher) from a hydrocarbon reformer effluent have typically used a layered adsorbent approach. Such H2PSA processing is disclosed in U.S. Pat. No.3,430,418 to Wagner. The first adsorbent layer is typically activated carbon used for removal of water, CO2 and CH4. The second adsorbent layer is typically a zeolite for removal of CO and N2 to the low levels necessary to generate a high purity H2 stream.
Other patents that use a layered bed approach to produce high purity H2 from reformer effluent include U.S. Pat. No. 3,986,849 to Fuderer et al., U.S. Pat. No. 4,964,888 to Miller; U.S. Pat. No. 5,133,785 to Kumar et al., and U.S. Pat. No. 6,027,549 to Golden et al. In all of these patents, the adsorber on-line (feed) time is on the order of 2 to 6 minutes. In the ""849 patent, there is an example with activated carbon as the sole adsorbent for purification of a H2-containing stream with 5.8% CH4 and 2.4% CO. The example suggests that activated carbon allows significant unwanted breakthrough of CO. Subsequent preferred examples all use layered beds of carbon followed by zeolite.
There is other art on the use of carbon only for H2PSA processes. U.S. Pat. No. 4,077,780 to Doshi teaches a PSA process for separating gas mixtures containing ammonia, argon, methane, nitrogen and hydrogen to recover both nitrogen and hydrogen. Activated carbon is the preferred adsorbent for this application, because it is desired to recover both N2 and H2 from the feed stream. Since carbon has a much lower N2/H2 selectivity than zeolites, it is the preferred adsorbent for simultaneous H2 and N2 recovery. The feed time taught by Doshi is 5 minutes.
U.S. Pat. No. 6,261,343 to Golden et al. teaches the use of activated carbon only PSA beds for the purification of H2 with O2 and/or Ar impurities in the feed stream. Since Ar and /or O2 are the most weakly adsorbed components in the H2-containing feed stream after H2, their breakthrough will determine the on-line time. The ""343 patent teaches that active carbon has improved O2 and Ar removal capability over zeolites and is therefore the preferred adsorbent when the H2 purity is limited by O2 and/or Ar breakthrough.
U.S. Pat. No. 4,077,779 to Sircar et al. teaches the use of active carbon only beds for production of H2 from CO2/H2 mixtures. Thus, the carbon is being used for bulk CO2 removal from H2. The feed time disclosed in the ""779 patent is 4 minutes.
The cost of hydrogen from integrated reformer/PSA systems is impacted by both the capital and operating costs of the system. Reducing the cycle time of the PSA can significantly reduce the capital cost of the PSA. As the cycle time decreases, the bed size also decreases resulting in a reduction in plant capital costs.
In addition, with the advent of fuel cell technology it is of considerable interest to develop micro-hydrogen generators. For example, a fuel cell powered car with an on-board reformer would require a small hydrogen purification unit. The ultimate goal is to develop as small a purification unit as possible to produce a given volume flow of hydrogen. The volume of bed required to produce a given flow of hydrogen can be termed a bed sizing factor with units of ft3 of bed/ft3 of H2/sec. Thus, it is desired to obtain a hydrogen purification system with as small a bed sizing factor as possible. In order to reduce the bed size of the hydrogen PSA to fit under the hood of a car, fast cycles are required.
There are a number of patents that teach rapid cycle PSA processes. U.S. Pat. No. 6,231,644 to Jain et al. describes an improved air separation process utilizing a monolithic adsorbent material where the cycle time is 35 seconds. U.S. Pat. Nos. 6,176,897 and 6,056,804 to Keefer et al. disclose the operation of an ultra rapid PSA system using adsorbent laminate modules at a cyclic frequency of 100 cycles per minute, which corresponds to a cycle time of 0.6 second and even possibly as high as 600 cycles per minute (0.1 second cycle time). These patents illustrate rapid pressure adsorption systems operating at very short cycle times and necessitating novel adsorbent configurations, process cycle and mechanical device innovations. The end goal of these patents is to minimize the bed size required for production of a given flow of hydrogen (minimize the bed size factor). None of these patents teach the use of activated carbon as the preferred or sole adsorbent for RPSA applications.
Despite the foregoing developments, it is desired to provide improved RPSA systems and processes comprising the use of activated carbon for gas separation.
It is further desired to provide such improved RPSA systems and processes for producing hydrogen having a purity of at least 99.9%.
All references cited herein are incorporated herein by reference in their entireties.
The invention provides a pressure swing adsorption process for recovering a product gas from a feed gas, which process comprises: supplying a pressure swing adsorption apparatus including an adsorbent composition containing activated carbon as a major ingredient, wherein the adsorbent composition and said apparatus are substantially free of zeolite adsorbents; feeding a feed gas into the pressure swing adsorption apparatus during a feed period not exceeding 20 seconds; and recovering the product gas from the pressure swing adsorption apparatus.
Also provided is an apparatus for performing the process of the invention.
The process and apparatus are particularly suitable for use with fuel cells and in other applications requiring compact, rapid cycling systems for producing high purity hydrogen.